Prolonged inhibition of respiratory neural activity elicits a long-lasting increase in phrenic nerve amplitude, known as inactivity-induced phrenic motor facilitation (iPMF). Facilitation also occurs transiently in inspiratory intercostal nerve activity following inactivity (iIMF). Atypical PKC activity in the cervical spinal cord is necessary for iPMF and iIMF, but the site and relevant PKC isoform are unknown. Here, we used RNA interference to test the hypothesis that the atypical PKCζ isoform within phrenic motor neurons is necessary for iPMF, but PKCζ within intercostal motor neurons is unnecessary for transient iIMF. Intrapleural siRNA injections were made in rats to knock down phrenic and intercostal motor neuron PKCζ mRNA (siPKCζ). Control rats received non-targeting siRNA (NTsi) or siPKCθ; PKCθ is required for other forms of respiratory motor plasticity. Phrenic nerve and external intercostal (T2) EMG activity were measured in anesthetized, mechanically ventilated rats exposed to 30 min of respiratory neural inactivity (i.e. neural apnea) from modest hypocapnia, or a similar duration without neural apnea (time control). Phrenic burst amplitude increased from baseline with NTsi (68±10%) and siPKCθ (57±8%) 60 min post-neural apnea versus time controls (-3±3%). In contrast, siPKCζ virtually abolished iPMF (5±4%). While iIMF was transient in all groups, siPKCζ attenuated iIMF 5 min post-neural apnea (50±21%) vs NTsi (97±22%) and siPKCθ (103±20%). Neural inactivity elevated phrenic, but not intercostal responses to hypercapnia--an effect blocked by siPKCζ. We conclude phrenic motor neuron PKCζ is necessary for long-lasting iPMF, whereas intercostal motor neuron PKCζ contributes to, but is not necessary for transient iIMF.
Keywords: apnea; homeostatic plasticity; neural control of breathing; phrenic; plasticity.